The present invention relates to offset printing presses and the like and, more particularly, to an improved paper support bar for mounting on the chain delivery system of a printing press comprising an elongated hollow cylinder of high-density polyethylene, polypropylene, or other rigid, non-brittle material containing electrically conductive carbon black, antistatic agent, or other triboelectric charge directing agent; a pair of metal plugs secured into respective ends of the cylinder, each plug having a longitudinal bore therethrough; a pair of longitudinal bar members disposed as a sliding fit within respective ones of the bores, the bar members each having an outer end adapted to clip onto the links of one of the chains and an inner end; and, a pair of compression springs disposed within the cylinder around respective ones of the bar members between the inner end thereof and the adjacent plug.
The critical portions of an offset printing press as wherein the present invention is intended for use are shown in simplified form in a side view in FIG. 1. To effect the printing operation, there are three cylinders moving in contacting relationship to one another. The plate or master cylinder 10 contacts the blanket cylinder 12 which, in turn, contacts the impression or backup cylinder 14. The impression cylinder 14 includes gripping fingers (not shown). As a sheet of paper 16 is fed from the supply stack 18 as by the feed roller 20, the gripping fingers on the impression cylinder 14 grip the leading edge of the paper sheet 16, thus pulling the paper sheet 16 between the blanket cylinder 12 and impression cylinder 14 on the surface of the impression cylinder 14 as it rotates in the direction indicated by arrow 22.
As best seen in FIG. 2, a pair of parallel, spaced chains 24 are mounted on sprockets 26 which, in turn, are mounted on driven shaft 28. The driven shaft 28 is mounted parallel to and close adjacent the surface of the impression cylinder 14. As a result, the chains 24 move towards, past, and then away from the surface of the impression cylinder 14, as indicated by the arrows 30 in FIG. 1. The chains 24 are used to effect the removal of printed paper sheets 16 in a manner shown in simplified form in FIGS. 4-7.
Referring first to FIG. 4, the paper sheet 16 is gripped along its leading edge by the fingers (not shown) previously described as being incorporated into the impression cylinder 14. The fingers are opened and closed by the drive mechanism for the impression cylinder 14 in a manner which is unimportant to the present invention. The fingers are located in the area indicated by the arrow 32. A gripper bar 34 is mounted between the chains 24 so as to move in combination therewith. The gripper bar 34 is shown in greater detail in FIG. 8. The gripper bar 34 contains a plurality of hinged gripping fingers 36 which can be opened and closed in combination by the movement of arm 38 which, in turn, is moved by roller 40 moving along a camming surface (not shown).
Returning to FIG. 4, as the paper sheet 16 moves out from between the point of contact between the blanket cylinder 12 and the impression cylinder 14, the top surface 42 is imprinted with an ink image as a result of the printing process. As the paper sheet 16 moves towards the driven shaft 28, the gripper bar 34 is moving towards the impression cylinder 14. That is, the leading edge of the paper sheet 16 and the gripper bar 34 are moving towards a point of close proximity. When they come together as shown in FIG. 5, the fingers on the impression cylinder 14 release their grip on the sheet of paper 16 while, simultaneously, the gripping fingers 36 of the gripper bar 34 seize the leading edge of the paper sheet 16. As the impression cylinder 14 and sprockets 26 continue to rotate, the paper sheet 16 is pulled from the surface of the impression cylinder 14 and away from it by the gripper bar 34. Finally, as shown in FIG. 7, with its momentum in the direction of arrow 44, the paper sheet 16 is released by the gripper bar 34 causing it to strike stripper finger 47 and fall into the takeup tray 46.
Returning now to FIGS. 2 and 3, as the paper sheet 16 is removed from the surface of the impression cylinder 14 by the gripper bar 34, it is important that the paper sheet 16 be kept close adjacent the surface of the impression cylinder 14 during the change in direction so as to prevent creasing of the paper 16 and/or smearing of the freshly deposited ink on the top surface 42. To this end, one or more so-called "star wheels", generally indicated as 48, were mounted on the driven shaft 28 between the sprockets 26. The star wheels 48 were held onto the driven shaft 28 by set screws 50. The periphery of the star wheels 48 were provided with projections 52 so that the top surface 42 of the paper sheets 16 would be contacted only at discreet points in an attempt to reduce or eliminate "tracking", i.e., imperfections in the printed top surface 42 resulting from contacting the still-wet ink before it has an opportunity to dry and redepositing it in other areas of the printed copy sheet. The tracking problem is particularly acute where there are large areas of solid color. The prior art star wheels 48 have two major problems. First, they are prone to produce tracking. Second, and more important, they are prone to falling off of the driven shaft 28 as a result of the loosening of the set screw 50, often resulting in catastrophic damage to the impression cylinder 14. In an effort to remove the troublesome star wheels, it was found many years ago that one could stretch rubber bands between paperclips clipped onto the chains 24. The rubberbands, however, were prone to breakage causing the paperclips to fall into the mechanism. Their use, however, did demonstrate that horizontal support members carried by and between the chains 24 could effectively replace the star wheels. Using that basic idea, one manufacturer of offset printing equipment (Offset Engineering Co., Inc. of Encino, Calif. has provided support bars such as that generally indicated as 54 in FIG. 8. These "Sheetech" prior art bars 54 are sized for the distance between the two chains 24, are of metal, and have spring PG,8 loaded clips 56 on the ends thereof for gripping the chains 24. There are three gripper bars 34 on the typical press and the prior art Sheetech bars 54 are usually positioned with from one to three bars 54 between each gripper bar 34. Additionally, each bar 54 has a plurality of plastic three or four pointed members 55, as shown in FIG. 13, movably press fit thereon along the length thereof. The points of the members 55 create small areas of contact in the same manner as the star wheels and or the same purpose. Each of the points of the members 55 is of a different length so as to be able to contact the paper in a different manner. Unfortunately, the prior art Sheetech bars 54 are not a significant improvement over the old star wheels. The members 55 still produce tracking and, therefore, they must be adjusted for each new printing job so as to contact the printed paper at points where there is no ink on the surface. Additionally, despite their gripping ends, the Sheetech bars 54 are not immune to becoming dislodged from the chains 24 and, should they do so, they can fall into the impression cylinder 14, again causing catastrophic damage thereto.
Wherefore, it is the object of the present invention to provide a support bar for use on offset printing presses and the like which will virtually eliminate tracking without the necessity for constant readjusting and which will be harmless to the impression cylinder and other working parts of the press should it become dislodged from its working position .